Euro code - LTB checks & 2 bay portal frame

[OJM]

Hi all, I’ve got two questions.

Firstly when designing a beam for lateral torsional buckling I always thought you assume it buckled about its weakest axis (I.e. use Izz) even if it is loaded parallel to its strongest axis (the MCr equation uses Iz) Is this the case? As in the blue book the main values give you buckling about its strong axis as the default values of Mb,Rd, however I thought buckling about its ZZ axis would be the limiting factor?

Secondly I am trying to look at removing existing X tension bracing in a standard single storey steel frame and replacing with portal used bays. I can do this easily using software etc... but to do this manually I am looking at two moment frames connected together ( 3 cols and 2 beams) out of 7 total bays. I assume this would be a sway frame as not braced and therefore could you use the standard moment distribution method based on stiffness and fixed end moments? As there are no beam loads only a horizontal wind load at the roof level and some column loads where trusses bear onto them. The normal Fixed end moments don’t seem to cover loads going into the joints. And resolving it is too tricky as it’s statically indeterminate? Where would you recommend me looking for this problem?

Very much appreciate your responses!

 

[Eagleee]

It is difficult to understand what you are asking, at least for me, maybe indicating some confusion on your part on the nature of the LTB concept (and maybe why you have received no replies here). LTB engages both lateral stiffness (Iz) and rotational stiffness (Iw, It). All these 3 terms go into the calculation of critical moment for a beam (not just Iz as you write above, though maybe in your specific case the influence of the others is negligible). twist + lateral displacement for a beam subjected to major axis bending is equivalent to what lateral displacement is for a column subjected to compression. in both cases, these stability considerations reduce the capacity calculated considering a cross-section analysis. Mb,Rd is the reduced YY bending capacity, with this reduction depending on Iz, It, Iw (among others).

For the second one, it is even less clear what you are asking...